Optical designs for high-efficacy white-light emitting diodes

ABSTRACT

A method for increasing the luminous efficacy of a white light emitting diode (WLED), comprising introducing optically functional interfaces between an LED die and a phosphor, and between the phosphor and an outer medium, wherein at least one of the interfaces between the phosphor and the LED die provides a reflectance for light emitted by the phosphor away from the outer medium and a transmittance for light emitted by the LED die. Thus, a WLED may comprise a first material which surrounds an LED die, a phosphor layer, and at least one additional layer or material which is transparent for direct LED emission and reflective for the phosphor emission, placed between the phosphor layer and the first material which surrounds the LED die.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. Section 120 ofco-pending and commonly-assigned U.S. Utility patent application Ser.No. 12/163,510, filed on Jun. 27, 2008, by Frederic S. Diana, Steven P.DenBaars, and Shuji Nakamura, entitled “OPTICAL DESIGNS FORHIGH-EFFICACY WHITE-LIGHT EMITTING DIODES” attorneys' docket number30794.232-US-U1 (2007-503-2), which application claims the benefit under35 U.S.C. Section 119(e) of co-pending and commonly-assigned U.S.Provisional Patent Application Ser. No. 60/946,652, filed on Jun. 27,2007, by Frederic S. Diana, Steven P. DenBaars, and Shuji Nakamura,entitled “OPTICAL DESIGNS FOR HIGH-EFFICACY WHITE-LIGHT EMITTING DIODES”attorneys' docket number 30794.232-US-P1 (2007-503-1), both of whichapplications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to semiconductor light-emitting diodes (LEDs)and in particular presents optimized optical designs aimed at improvingthe luminous efficacy of white-light emitting diodes (WLEDs) used forlighting applications.

2. Description of the Related Art

Currently, there exists a multitude of WLED packaging configurations inthe state of the art. The term packaging encompasses a broad technicalscope. With respect to LEDs it refers to all the fabrication steps whichfollow wafer processing: dicing of LED chips, transfer of these ontoheaders or supports to provide electrical injection and heat-sinking,integration of secondary light-emitting species, and encapsulation withtransparent materials to enhance light extraction and allow deviceprotection and passivation (this sequence of steps could be performed indifferent order).

In the following, the term LED die is used to refer to a semiconductorchip, which includes electroluminescent primary light-emitting species(such as quantum wells or any other type of semiconductorheterostructures). The term phosphors refers to the optically pumpedsecondary light-emitting species, without loss of generality.

Out of the large number of ways of packaging WLEDs, there are mostly twomain configurations for phosphors integration: phosphors-on-chip andremote-phosphors configurations.

As FIG. 1 shows, in the phosphors-on-chip configuration 100, thephosphors 102 are placed in the direct vicinity of the semiconductor LEDdie 104, either as powders coating the chip 104 or as mixtures withresins surrounding the chip 104, with different concentrations andgeometrical dimensions. The LED die 104 is usually fixed on a reflectingheader or LED cup 106, providing electrical injection and heat-sinking,and embedded in a transparent epoxy 108 (resin, silicone, etc.).

The use of a transparent epoxy 108 allows an increase in the lightextraction efficiency of the device 104 because transparent epoxies havea higher index of refraction (n) than air in the near-ultraviolet,visible, and infrared wavelength ranges. Light which is emitted fromwithin the high-index semiconductor chip 104 (usually n>2 for mostsemiconductors) can escape outside only if the light's angle ofincidence inside the chip 104 is within the light escape cone, that is,below the critical angle for total internal reflection (TIR) θ_(c).θ_(c) depends on the index of refraction of the medium which surroundsthe LED die 104, n_(out), and the refractive index of the LED die 104,n_(in): θ_(c)=arcsin(n_(out)/n_(in)). The value of θ_(c) increases from24° to 35° as the medium exterior to a GaN LED die 104 (n_(in)=2.5) ischanged from air (n_(out)=1) to a common transparent epoxy 108(n_(out)=1.45).

For this increase in light extraction to occur, it is necessary that (1)the LED chip 104 and the epoxy 108 be in close contact (if even a thinlayer of air or vacuum separates the LED chip 104 from the epoxy 108,the potential increase in light extraction is cancelled), and (2) theinterface 110 between epoxy 108 and external medium (usually air) becurved or shaped, such that the greatest portion of the light raysextracted from the LED die 104 and phosphors 102 impinge onto thisinterface 110 at incidence angles much smaller than the critical anglefor total internal reflection: θ_(c)˜43° for epoxies 108 with an indexof refraction ˜1.45.

The whole LED die+cup+resin+phosphors assembly can therefore be placedin a hemispherical or dome-shaped material 112 (an optic), which can bemade out of a transparent epoxy or optical glass. With this shape, mostlight rays are incident at nearly 0° (θ_(c)˜43°), the angle at whichreflectance is minimized.

However, in the phosphors-on-chip configuration 100, light rays emittedfrom the phosphors 102 downwards (that is, emitted towards the LED die104 and header or LED cup 106) are partly absorbed by the LED metalcontacts and the doped semiconductor layers necessary to electricallyinject the primary emitting species included in the LED chip 104.Indeed, in order for these light rays to escape, they must propagatedownwards across the LED die 104, be reflected upwards by a sufficientlyreflective layer (either included on the bottom surface of the LED die104 or on the LED cup 106), propagate across the LED die 104 once again,and then through the phosphors layer 102, without being absorbed. Inaddition, light rays emitted by the phosphors 102 upwards, as well asthe primary light extracted from the LED die 104 used to optically pumpthe phosphors 102, undergo multiple reflection, refraction, andscattering events, due to the presence of large phosphor particles(usually larger than 5 μm in diameter), wherein the phosphor particleshave indices of refraction (n˜1.75 for rare-earths-doped YAG phosphors)which are usually different from the refractive index of the matrix inwhich the phosphor particles are embedded (n˜1.45). These scatteringevents increase the probability of absorption of light. Finally, in thisconfiguration 100, phosphors 102 are in direct contact with the hightemperature of the LED junction 114 under operation, which can reachtemperatures larger than 150° C. At such elevated temperatures, thedegradation rate of phosphors 102 is increased and their internalquantum efficiency is usually reduced.

These negative effects are partly eliminated by using theremote-phosphors configuration 200. In this configuration 200, thephosphors 202 are separated from the LED die 204, that is, they areplaced at least 200 μm away from the upper surface 204s of the LED die204, as depicted on FIG. 2. This configuration 200 allows an increase inthe overall luminous efficacy of WLEDs, by reducing the probability oflight absorption caused by scattering and absorption by metals and dopedsemiconductor layers. In addition, this configuration 200 places thephosphors 202 away from the region of elevated temperature, which is inthe vicinity of the LED die 204 under operation. The resin 206, LED cup208, and optic 210 are also shown in FIG. 2.

There are different possibilities for phosphors application in a remotephosphors configuration. FIG. 3 shows an example of a remote phosphorsconfiguration 300, wherein the phosphors 302 are coating the dome-shapedlight-extracting optic 304. The LED die 306, resin 308, and LED cup 310are also shown in FIG. 3.

The previous configurations, and all of their possible geometries, arenot restricted to be used in combination with the dome-shapedlight-extracting optic 112, 210, or 304. Inverted-truncated cones canalso be used instead to obtain similar light-extraction performance. InFIG. 4, the phosphors 400 are placed on top of an inverted-truncatedcone-shaped optic 402, which could be formed from a resin or opticalglass, while in FIG. 5, the whole assembly 404 of FIG. 4 (comprising thephosphors 400, optic 402, LED die 406 and LED cup 408) is capped byanother dome-shaped optic 410. The use of such truncated cone 402 is apossible alternative to the use of hemispherical-shaped optics, becauselight rays extracted from the LED die 406 impinge on the sidewalls 412a, 412 b of the cone 402 at angles larger than θ_(c), and hence aretotally internally reflected and can escape upwards through the uppersurface 414 of the optic 402.

Although the previous geometries of the remote-phosphors configurationcan help to reduce light absorption, there is still a need in the artfor improving light extraction efficiency by making use of alternativepackaging geometries.

Indeed, when phosphors 202, 400 are placed inside an LED cup 208, oradjacent an inverted cone 402 made of resin, as shown in FIG. 2 and FIG.5 respectively, the phosphors 202, 400 are embedded in and/or surroundedby a material 206, 402, 210, and 410 with index of refraction of about1.45. The symmetry of this configuration implies that light emitted frominside the phosphors layer 202, 400 propagates in nearly equal amountsupwards and downwards. Therefore, about 50% of the light emitted by thephosphors 202, 400 must undergo reflection at the bottom 208 b of theLED cup 208 before having the possibility to escape outside. Part ofthis light is therefore absorbed in the process. Actually, the amount oflight emitted by the phosphors 202, 400 downwards is slightly largerthan that emitted upwards; indeed, the primary light rays (i.e. raysemitted by the LED die 406, 204) are incident from the bottom 416 of thephosphor layer 202, 400 (i.e. the part of the phosphor nearest the LEDdie 204, 406), and therefore more light is emitted by the phosphorslocated near the bottom 416 of the phosphor layer 202, 400 than thosephosphors located near the top 418 of the phosphor layer 202, 400. As aconsequence, the scattering of secondary light (i.e. light emitted bythe phosphor 202, 400) propagating upwards is larger than that ofsecondary light propagating downwards.

With the other remote-phosphors geometry (as shown in FIG. 3 and FIG.4), the situation is nearly similar, and in addition anothercomplication stems from the fact that a smooth upper interfaceseparating the phosphors layer and air is present near the phosphors. AsFIG. 6 shows, the smooth interface 600 only allows the light rays 602(emitted by a light source 604 such as a phosphor particle) incident atangles smaller than θ_(c) 606 inside the escape cone 608 to be extractedin the external medium 612 (usually air). The rest of the light rays614, 616 are either totally internally reflected (totally internallyreflected secondary light ray 614), or propagate downwards (transmittedsecondary light ray 616) through transparent interfaces, such as 618,back towards the internal medium/optic 620 and the LED die 622, inuseless directions, and towards regions where the probability for lightabsorption is not negligible. FIG. 6 also shows a possible trajectoryfor a transmitted primary light ray 624 emitted by the LED die 622, andtransmitted into the phosphor layer 626 where the ray 624 interacts witha light source 604 such as a phosphor particle inside the phosphor layer626. Thus, there is a need in the art for improved packagingconfigurations to enhance the light extraction from phosphor layers, forexample. The present invention satisfies this need.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesthe design principles and examples of high-efficacy white-light emittingdiode (WLED) lamps by using several emitting species and optimizedpackaging configurations. A WLED lamp is comprised of an LEDsemiconductor chip, a header to support the chip and allow electricalinjection and heat-sinking, and encapsulating materials possiblycomprising secondary emitting species providing light with differentwavelengths. The LED chip is usually comprised of a substrate, a bufferlayer grown on the substrate (if such a layer is needed), and anelectrically-injected active region comprising primary emitting species.Other optically active substances comprise secondary emitting specieswhich are optically pumped by the light of the primary emitting speciesand re-emit light of different wavelengths. The secondary emittingspecies are generally comprised of light-down-converting particulatephosphors embedded in a passive transparent matrix. The presentinvention is related to the integration of optical elements and designswhich allow the demonstration of white LED lamps with overall luminousefficacies larger than 100 lm/W.

The present invention further discloses a White Light Emitting Diode(WLED), comprising a Light Emitting Diode (LED) for emitting primarylight; a secondary emitting species optically coupled to the LED, foremitting secondary light comprising one or more wavelengths differentthan a wavelength of the primary light; and at least one opticallyfunctional interface positioned between the secondary emitting speciesand the LED, wherein the optically functional interface is at leastpartially transparent for the primary light incident from the LED and isat least partially reflective for the secondary light incident from thesecondary emitting species.

The WLED may comprise more than one optically functional interface, forexample, two (or more) optically functional interfaces. The opticallyfunctional interface may be positioned at a distance from the LED atleast equal to a lateral extent of the LED. The optically functionalinterface may comprise a first material. The optically functionalinterface may comprise a surface of the first material and the materialmay be positioned such that its refractive index increases totalinternal reflection of the secondary light inside the secondary emittingspecies at the optically functional interface.

The secondary emitting species may be a phosphor layer emitting thesecondary light when optically pumped by the primary light, the secondmaterial may at least partially surround the LED and be positioned suchthat the second material's refractive index reduces the primary light'stotal internal reflection inside the LED at an interface between the LEDand the second material; and the first material may be positionedbetween the secondary emitting species and the second material.

The first material's refractive index may be smaller than the phosphorlayer's refractive index. One of the optically functional interfaces maycomprise a third material, wherein the third material is positionedbetween at least some of the second material and the first material.

The WLED may comprise an LED die on a header or LED cup; the secondmaterial may comprise a dome shaped outer surface, wherein the LED dieis encapsulated by the second material and the header or LED cup; andthe first material may cap the outer surface and the phosphor layer maycap the first material. Or the WLED may comprise an LED die on a headeror cup, the LED die may be encapsulated by the second material and theheader or LED cup, the third material may comprise a dome shaped outersurface; the first material may cap the outer surface of the thirdmaterial; the LED may be positioned in the LED cup such that the LED'sfield of view comprises an entirety of the outer surface of the thirdmaterial, and the LED cup may reflect the primary light towards thephosphor.

The first material may be an air gap comprising a thickness much largerthan a wavelength of the primary light. The second material may beresin, epoxy, silicon or glass.

The LED header may be on a mounting fixture, and the phosphor layer maybe mounted to the mounting fixture via a reflective material.

Emission from the phosphor layer may be a yellow light emission and theLED die may be a III-Nitride-based LED die emitting blue light.

The WLED may be for emitting white light with an overall luminousefficacy of 100 lm/W and a packaging efficacy of 100 lm/W greater. TheWLED may be for emitting white light with a color rendering index of atleast 60.

The present invention further discloses a method for fabricating a WhiteLight Emitting Diode (WLED), comprising providing one or more opticallyfunctional interfaces, between a secondary emitting species and an LED,to reflect secondary light emitted by the secondary emitting speciesaway from the LED, wherein the one or more optically functionalinterfaces are at least partially transparent for primary light incidentfrom the LED and at least partially reflective for the secondary lightincident from the secondary emitting species, and the secondary lightcomprises one or more wavelengths different than a wavelength of theprimary light.

One of the one or more optically functional interfaces may be positionedat a distance from the LED at least equal to a lateral extent of theLED.

The LED may be an LED die and the secondary emitting species may be aphosphor layer, and the method may further comprise encapsulating theLED die with a first material and a header or LED cup, wherein the firstmaterial comprises a dome shaped outer surface and is positioned suchthat the first material's refractive index reduces the primary light'stotal internal reflection inside the LED at an interface between the LEDand the first material; and capping the outer surface with the one ormore optically functional interfaces comprising a second material,wherein the second material has a refractive index smaller than thephosphor layer's refractive index and the one or more opticallyfunctional interfaces reflect the secondary light away from thematerial.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a cross-sectional schematic of a phosphors-on-chipconfiguration.

FIG. 2 is a cross-sectional schematic of a remote-phosphorsconfiguration.

FIG. 3 is a cross-sectional schematic of a remote phosphorsconfiguration, wherein the phosphors are coating the dome-shapedlight-extracting optic.

FIG. 4 is a cross-sectional schematic of a remote phosphorsconfiguration, wherein the phosphors are placed on top of aninverted-truncated cone-shaped optic.

FIG. 5 is a cross-sectional schematic showing an assembly capped byanother dome-shaped optic.

FIG. 6 illustrates how an interface only allows light rays, emitted byphosphors, incident at angles smaller than θ_(c) inside the escape coneto be extracted in the external medium (usually air).

FIG. 7 is a cross-sectional schematic of an LED wherein phosphors areplaced sufficiently far away from the LED die, with the introduction ofmultiple optically functional interfaces between the die and thephosphors layer and between the phosphors layer and the outer medium.

FIG. 8 shows the calculated optical characteristics (Reflectance R,Transmittance T, and Amplitude A) of a dichroic mirror, obtained bystacking a 100 nm-thick silicon dioxide layer and a 80 nm-thick siliconnitride layer five times with other termination layers, wherein thecalculation accounts for the stack being surrounded by resin and air asif it were positioned at the interface in FIG. 7, with air between theoptic and the internal medium.

FIG. 9 shows a comparison of TE-polarized plane wave transmittances intoair through a planar (thick black line) and a patterned (thin grey line)interface, as a function of the incidence angle.

FIG. 10 is a cross-sectional schematic of an LED die placed in a highlyreflective cup, encapsulated by a first transparent material, itselfcapped by a hemispherical transparent optic with similar index ofrefraction to limit reflection at the interface.

FIG. 11 is a cross-sectional schematic showing a phosphors layer whichis not uniform in thickness, but can be made thicker on top and thinnerat the sides of the hemispherical dome.

FIG. 12 is a cross-sectional schematic illustrating the LED cup is mademore or less shallow to provide a wider or narrower angle of view and todecrease the amount of resin that lies just above the LED die.

FIG. 13 is a cross-sectional schematic illustrating another example,wherein the LED cup is made more or less shallow to provide a wider ornarrower angle of view and to decrease the amount of resin that liesjust above the LED die.

FIG. 14 is a cross-sectional schematic of an LED die surrounded by aconical-shaped transparent material, to take advantage of the totalinternal reflection effect, instead of using a metallic reflector whichabsorbs parts of the light.

FIG. 15 is a cross-sectional schematic of the optic shaped as aninverted-truncated cone.

FIG. 16 is a cross-sectional schematic of a multiple packaged LEDsmounted in a highly reflective frame, wherein the phosphors layer withfunctionalized interfaces is mounted such as to surround all the LEDs.

FIG. 17 is a schematic illustrating a process for fabrication of anoptimized phosphors layer geometry and configuration.

FIG. 18 illustrates another process for fabrication of an optimizedphosphors layer geometry and configuration.

FIG. 19 shows a cross-sectional schematic of a device using acommercially available blue LED chip.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

Current WLEDs combine electroluminescent semiconductor chips tooptically pump secondary light-emitting substances, such as inorganicphosphors (for instance, blue light can pump yellow-emitting phosphorsto produce white light). The present invention provides novel packagingconfigurations allowing overall luminous efficacies of WLEDs to begreater than 100 lumens per Watt (lm/W), provided that bothlight-emitting substances possess sufficiently high internal quantumefficiencies.

Technical Description

The present invention aims at increasing WLEDs' overall luminousefficacy by reducing the probability of primary and secondary lightabsorption. The main optical designs aimed at improving LED luminousefficacy are illustrated in FIG. 7. The common point of all theseschemes is to reduce to a minimum the probability of light absorption.Therefore, the phosphors light source 700 (or phosphor layer 702) areplaced sufficiently far away from the LED die 704 (at a distance 706 atleast equal to the lateral extent 708 of the LED die 704). In thisregard, the present invention introduces multiple optically functional(or modified) interfaces (710, 712, 714, 716) between the die 704 andthe phosphors layer 702 and between the phosphors layer 702 and theouter (or external) medium 718.

At least one of the interfaces, e.g. 710, located between the phosphorslayer 702 and the LED die 704 or internal medium 720 should provide ahigh reflectance for the secondary light emitted by the phosphors 700downwards (to reflect reflected secondary light rays 722), and a hightransmittance for the primary light 724 emitted by the LED die 704 (totransmit transmitted primary light ray 724). At least one of theseinterfaces, e.g. 710, should be a partial reflector, or dichroic mirror,which would provide a very high transmittance for light with wavelengthscomprised in the near-UV to blue range (emitted by the LED 704), and ahigh reflectance for light with wavelengths comprised in the green,yellow, or red portions of the visible spectrum (emitted by thephosphors 700). The interface 710 may be between a surface of an optic726, such as a dichroic mirror, and the internal medium 720.

Such an optical element 726 can be formed by using a dielectricmultilayer, as is commonly done to form distributed Bragg reflectors(DBRs). FIG. 8 shows the calculated optical characteristics of one suchdichroic mirror, obtained by stacking a 100 nm-thick silicon dioxidelayer and a 80 nm-thick silicon nitride layer five times with othertermination layers. In this calculation, the stack is surrounded byresin and air as if it was positioned at interface 710 in FIG. 7, withair between the optic 726 and the internal medium 720. Several curvesare shown on this plot. The curves with solid lines are the stack'sreflectance (R) vs. wavelength characteristics for five different anglesof incidence (wherein the 5 solid curves, from right to left in FIG. 8,represent R for angles of incidence 0°, 10°, 20°, 30°, and 40°,respectively, as indicated in the legend). The curves with dash-dottedlines are the stack's transmittance (T) vs. wavelength characteristicsfor five different angles of incidence (wherein the 5 dash-dottedcurves, from right to left in FIG. 8, represent T for angles ofincidence 0°, 10°, 20°, 30°, and 40°, respectively). The line 800 is ameasured spectrum of a WLED combining blue and yellow light underoperation. The calculation shows that it is relatively simple to formthe desired optical element 726, with the only necessary condition thatthe incident light rays extracted from the LED die 704 would have to bedirected such that they would not have incidence angles larger than 40°on the dichroic mirror 726. An alternative to this could be to surroundthe dichroic mirror 726 in materials of similar indices of refraction(n˜1.45-1.5), but with a slightly modified multilayer structure. Currentelectron-beam or plasma deposition technologies can allow large scalefabrication of such multilayers.

Another type of partial reflector can be used by introducing one or moreintermediate layers with a low index of refraction, such as representedby 728 in FIG. 7. Such a low-index layer 728, for example, an air gap ofthickness much larger than the wavelength of light, can introduce auseful total internal reflection for light emitted by phosphors 702,while at the same time keeping a very high transmittance for lightemitted by the LED die 704. Indeed, light emitted by the phosphors700,702 at angles larger than θ_(c) (˜43° for air/resin or glassinterface) is totally internally reflected 730 (reflected secondarylight ray), giving a solid-angle-averaged reflectance for such interface714 of 75%, assuming an isotropic light source 700. That is, 75% of thelight emitted downwards would be reflected back up simply by using anair gap 728, instead of 0% if the phosphor layer 702 is not opticallydecoupled from the internal medium 720. Previously, downwardspropagating light rays could only be reflected back up by the LED die704 and highly reflective LED cup 732 where losses occur. In addition,if this layer 728 is placed sufficiently far from the LED die 704 (at adistance 706 b at least equal to the lateral extent 708 of the LED die),and if the layer 728 is shaped as a hemispherical layer, all light rays,e.g. 724, emitted from the LED die 704 cross this layer 728 at nearlynormal incidence, and thus with minimal reflectance.

The phosphor particles 700 do not backscatter light efficiently becausetheir sizes are usually much larger than the wavelength of light, andbecause of the fact that the index of refraction mismatch Δn betweenphosphors 700 and the matrix (wherein the phosphor layer 702 comprisesthe matrix and the phosphors 700) where they are embedded is not usuallyvery large (Δn˜0.3), and reflectance at normal incidence scales likeΔn².

In order for the light emitted by the phosphors 700 not to be guidedinside the layer 702 (thereby forming transmitted secondary light ray734), the upper interface 716 must be optically functionalized to becomeas transmissive as possible in both angular and spectral domains. Onepossible solution is to coat the smooth upper surface 736 of thephosphors layer 702 with an antireflection coating. Another solution isto texture the upper surface 736 with either a periodic, quasi-periodic,or quasi-random pattern. Such patterns can indeed be used to frustrateor partly cancel the total internal reflection that would occur at thatinterface 716 if it the surface 736 were left smooth.

To illustrate this effect, FIG. 9 shows a comparison of TransverseElectric (TE)-polarized plane-wave transmittances into air through aplanar (thick black line 900) and a patterned (thin grey line 902)interface (e.g. 716), as a function of the incidence angle. The patternconsists of a square surface modulation between air (e.g. externalmedium 718) and a transparent material with n=1.45 (e.g. 702) from whichthe plane-wave is incident, with a periodicity of 4 μm, a depth of 2 μm,and a filling factor of 40%. If the transmittance is assumed to beisotropic in-plane, then patterning induces an increase in lightextraction of 52%, as compared to the planar interface case (this numberis obtained by integrating the angular transmittance curves over thesubtended solid-angle). Although the transmittance of the patternedinterface 902 is slightly lower than that of the planar interface 900 atlow angles of incidence, the transmittance of the patterned interface902 becomes much larger than the transmittance of the planar interface900 at angles larger than the angle for total internal reflection(θ_(c)˜43°), and these angles are associated with a much larger portionof solid-angle.

Such patterning can be obtained in various ways. One method is to formthe phosphors layer 702 from a textured mold. Another method would be tosimply closely pack the phosphor particles 700, embedding them in atransparent matrix by molding, and removing the first few tens ofmicrons of transparent matrix by performing a selective etch, subsequentto unmolding.

The interface 712 between the intermediate layer 728 and the optic 726could be coated with another partial reflector or dichroic mirror, suchas described above, or with an antireflection coating. A simpleantireflection coating can be comprised of a quarter-wave-thicktransparent material, of index of refraction intermediate between airand resin, such as magnesium fluoride (n˜1.38).

Finally, the LED cup surface 738 should be as reflective as possible,and could be composed of a specularly reflective or diffuse reflectivecoating (metallic, such as silver or aluminum, or non-metallic, such asTeflon or Barium Sulfate, etc.), the LED die 704 being fixed onto a goodheat-sinking material in all cases. FIG. 7 also shows a transmittedprimary light ray 740 being transmitted through the phosphor layer 702into the external medium 718. The interface 714 may also reflect primaryrays 730 emitted by the LED 704 which have been back scattered towardsthe LED 704.

FIG. 10 shows an embodiment of the present invention. The LED die 1000is placed in a highly reflective LED cup 1002, encapsulated by a firsttransparent material 1004 (e.g. resin). The first transparent material1004 is itself capped by a hemispherical transparent optic 1006, whereinthe optic 1006 has optimized interfaces comprising surfaces 1006 a, 1006b and a similar index of refraction as the first transparent material1004 to limit reflection at the interface comprising surface 1006 a.This interface may be comprised of a dichroic mirror or dichroicreflector coating 1008 a on surface 1006 a, transmitting all primarylight rays and reflecting the secondary light rays emitted by thephosphors 1010. The phosphors layer 1010 surrounds the surface 1006 b ofthe optic 1006 and one or more intermediate layers 1012 of low-indexmaterial(s) lie in between the two media 1006 and 1010 (wherein thelayer 1012 may have a lower refractive index than media 1006 and 1010).The surface 1006 b of the optic 1006 facing the phosphors 1010 can becoated with an antireflection coating 1008 b so that the primary lightcan go through the interface (i.e. across surface 1006 b) without beingreflected. The upper surface 1014 of the phosphors layer 1010 can betextured, roughened or patterned to increase light extraction bylimiting total internal reflection. The LED cup 1002 may have a highlyreflective surface 1016.

In another embodiment shown in FIG. 11, a WLED 1100, for example, thephosphors layer 1102 is not uniform in thickness 1104 a, 1104 b, but canbe made with a thicker thickness 1104 a on top 1106 a and a thinnerthickness 1104 b at the sides 1106 b of the optic 1108, wherein theouter surface 1110 of the optic 1108 is shaped as a hemispherical dome.This thickness modification can be used to finely tune the angularresponse of the apparent white-light obtained by mixing primary andsecondary lights. The WLED 1100 of FIG. 11 further comprises one or moreintermediate layers 1112 between the phosphor layer 1102 and the optic1108 and coating the outer surface 1110 of the optic 1108 (wherein theoptic 1108 comprises optimized interfaces), an LED die 1114 in an LEDcup 1116, and resin 1118 encapsulating the LED die 1114.

In other embodiments 1200, 1300 shown in FIG. 12 and FIG. 13, the LEDcup 1202, 1302 is made more shallow (i.e. smaller depth 1204) or lessshallow (i.e. larger depth 1304) to provide a wider (FIG. 12) ornarrower (FIG. 13) angle of view and to decrease the amount of resin1206, 1306 that lies just above the LED die 1208, 1308. Since resin1206, 1306 yellowing can occur during the lifetime of the device 1200,1300, it may be advantageous to reduce the thickness 1210, 1310 of theresin 1206, 1306 between LED die 1208, 1308 and optic 1212, 1312 to aminimum. FIG. 12 also illustrates how the total amount of resin 1206 canalso be reduced (as compared to the amount of resin 1306 in FIG. 13) bya shallower LED cup 1202 (as compared to the LED cup 1302 in FIG. 13).Also shown in FIG. 12 and FIG. 13 are the intermediate layer(s) 1214,1314 and the phosphor layer 1216, 1316 with optimized interfaces.

In another embodiment 1400 shown in FIG. 14, the LED die 1402 issurrounded by a conical-shaped transparent material 1404 (comprisingresin, for example) to take advantage of the total internal reflectioneffect, instead of using a metallic reflector which absorbs parts oflight. The resin 1404 may sit in an LED cup 1406 and be capped by anoptic 1408 with optimized interfaces. The optic 1408 may be capped byone or more intermediate layers 1410 and phosphors 1412 with optimizedinterfaces, wherein the intermediate layers 1410 are between thephosphors 1412 and the optic 1408.

In another embodiment 1500 shown in FIG. 15, the resin/optic 1502 isshaped as an inverted-truncated cone. The LED die 1504 sits on an LEDcup 1506 and is covered by the optic 1502. The optic's top surface 1508is capped by one or more intermediate layers 1510 and phosphors 1512,wherein the intermediate layers 1510 are between the top surface 1508and the phosphors 1512.

In another embodiment 1600 shown in FIG. 16, multiple packaged LEDs 1602are mounted in a highly reflective frame 1604 and the phosphors layer1606 with functionalized interfaces is mounted such as to surround allthe LEDs 1602 (or intersect or interact with substantially all the lightrays emitted by the LED 1602).

The LED die can include optical elements to provide increased lightextraction, such as photonic crystals, roughened surfaces, patternedinterfaces and layers, together with optical confining layers and/orhighly reflective ones. The LED chip can also be formed into shapesdifferent from cubic or parallelepiped shapes, such as pyramidal ordiamond-like shapes. The LED chip can also be capped with a lightextraction “megacone,” for example, made out of ZnO. The LED chip mayalso cap a pedestal, wherein the pedestal is shaped so as to optimizelight extraction and wherein the pedestal is made out of a nearlyindex-matching material.

Fabrication Process

At least two possible processes of fabrication are possible for theoptimized phosphors layer geometry and configuration. One possibleprocess is shown in FIG. 17. In this process, the LED die (such as aFlip Chip (FC) LED chip 1700 is surrounded by an optic 1702 shaped bymolding directly around the LED die 1700 and header 1704. The LED die1700+header 1704+optic assembly 1702 is then attached onto a mountingfixture 1706. The phosphors layer 1708 with functionalized interfaces isthen subsequently attached onto the same mounting fixture 1706 via aphosphors support 1710. Highly reflective material 1712 may be placedbetween the phosphor support 1710 and the end 1714 of the phosphor layer1708 to prevent light which is totally internally reflected inside thephosphors layer 1708 from escaping without having maximally interactedwith the phosphors 1708. Highly reflective material 1716 may be placedbetween the header 1704 and the optic 1702 to reflect light (which istotally internally reflected inside the optic 1702) towards thephosphors 1708. One or more intermediate layers 1718 may be positionedbetween the optic 1702 (which may be a transparent dome with optimizedinterfaces) and the phosphors 1708.

Another process of fabrication, shown in FIG. 18, attaches an LED cup1800 to the optic 1802, for example, by curing an epoxy or silicone. Thephosphors layer 1804 with functionalized interfaces is then subsequentlyattached onto a lip 1806 or a flange of the optic 1802.

EXAMPLES (1) Using a Commercially Available Blue LED Chip

FIG. 19 shows a schematic of a device 1900. A blue LED lamp was formedby attaching a commercially-available CREE EZR450 LED die or chip 1902onto a silver header 1904 by using silver epoxy. Silver epoxy allows forvertical electrical conduction throughout the LED die 1902; morespecifically, it allows the injection of holes traveling from thesilicon submount to the p-doped GaN layers and active layers includingInGaN quantum wells. The top contact (n-contact) of this die 1902 wasconnected by a gold wire 1906 to the first (header) post 1908 of thesilver header 1904 via wire-bonding. The other (second) post 1910 of thesilver header 1904 was connected by another gold wire 1912 to the header1904 itself via wire-bonding. At this stage, the total radiant fluxΦ_(B) of the bare LED die 1902 on silver header 1904 was measured usinga calibrated integrating sphere to be 21.0 mW at 20 mA and 3.20 V (DCmeasurement). The wavelength of light at the emission peak was 447 nm.

A transparent resin (commercially available from General Electric (GE))was used to form a hemispherical-shaped dome 1914 encapsulating the LEDdie 1902 on silver header 1904 via a molding technique. The LED die 1902was placed slightly below (at a ˜2 mm distance 1916) the center 1918 ofthe dome 1914. The diameter 1920 of the dome 1914 was 7.5 mm. The totalradiant flux Φ_(B) of the encapsulated blue LED die 1902 on silverheader 1904 was then measured using a calibrated integrating sphere tobe 26.6 mW at 20 mA and 3.20 V (DC measurement). This shows that thetransparent silicone dome 1914 allowed a 25% increase in total radiantflux.

A yellow-emitting phosphor, provided by Mitsubishi Chemical Corp. (MCC),was then mixed with a transparent resin to form pastes with phosphorsconcentrations of 6%, 5%, or 3% by weight. The pastes were then moldedto form caps 1922 shaped as hollow hemispheres about 2 mm in thickness.

The caps 1922 (or phosphors layers) comprised of the pastes with 6%, 5%,or 3% phosphor concentrations were then each mounted, in turn, onto theencapsulated blue LED (comprising the transparent silicone dome 1914 andLED 1902 as described above), allowing an air gap 1924 of about 1 mm inthickness 1926 to be present between the upper surface 1928 of thetransparent dome 1914 encapsulating the blue LED die 1902 and the innersurface 1930 of the phosphor caps 1922. The caps' 1922 light outputcharacteristics were measured using a calibrated integrating sphere (DCmeasurements).

TABLE 1 Spectral characteristics of WLEDs 1900 comprised of the sameblue LED lamp capped with layers 1922 with three different phosphorconcentrations (conc.), wherein the LED die used in these measurementswas a CREE EZR450. Phosphor cap I V CCT conc. (%) (mA) (V) x y (K) CRI 65 2.88 .4004 .4715 4106 59.5 6 20 3.20 .4004 .4687 4088 59.9 5 5 2.88.3593 .4030 4732 63.3 5 20 3.20 .3593 .3993 4717 64.0 3 5 2.88 .3083.3122 6935 72.0 3 20 3.20 .3080 .3109 6972 72.2

Table 1 summarizes the spectral characteristics of the WLEDs 1900 withthe following symbols: I is the DC current flowing through the LED die1902, V is the applied voltage, x and y are the WLED spectra colorcoordinates, CCT is the correlated color temperature and CRI is thecolor rendering index associated with the measured WLED spectra. Notethe quasi-linear relationship between phosphor concentration and CCT.

TABLE 2 Main output characteristics of WLEDs 1900 comprised of the sameblue LED lamp capped with layers with three different phosphorconcentrations, wherein the LED 1902 die used in these measurements wasa CREE EZR450. Phosphor cap I V Φ_(B) Φ_(W) F_(W) η_(lum) η_(pack) conc.(%) (mA) (V) (mW) (mW) (lm) (lm/W) (lm/W) 6 5 2.88 7.1 4.686 1.942 134.9273.1 6 20 3.20 26.6 17.71 7.239 113.1 272.1 5 5 2.88 7.1 5.092 1.846128.1 259.6 5 20 3.20 26.6 18.62 6.819 106.5 256.4 3 5 2.88 7.1 5.2121.621 113.0 228.0 3 20 3.20 26.6 19.58 6.035 94.9 226.9

Table 2 summarizes the main output characteristics of the WLEDs 1900with the following symbols: I is the DC current flowing through the LEDdie, V is the applied voltage, Φ_(B) is the LED total blue radiant flux,and Φ_(W) and F_(W) are the WLED 1900 total radiant and luminous fluxes,respectively, as measured by the integrating sphere.

The main figure of merit of an WLED is its overall luminous efficacyη_(lum). η_(lum) is defined as the ratio of WLED total luminous fluxF_(W) to the total (electrical) power supplied to the device P=IV. Thisquantity is expressed in units of lm/W. The main figure of merit forquantifying the efficiency of a given packaging configuration's(including phosphors) ability to convert primary light and extract whitelight, is the packaging efficacy η_(pack). Here, η_(pack) is defined asthe ratio of the WLED's white luminous flux F_(W) to the WLED's blueradiant flux Φ_(B). η_(pack) is expressed in units of lm/W as well andis useful as a measure of packaging efficiency alone, because issuesassociated with electrical injection efficiency, or lack thereof, are donot affect η_(pack).

High overall η_(lum) were obtained with these WLEDs 1900:

128.1 lm/W<η_(lum)<134.9 lm/W at 5 mA, and

106.5 lm/W<η_(lum)<113.1 lm/W at 20 mA, for 4000 K<CCT<4900 K (a CCTrange which is appropriate for warm-white lighting applications). Highη_(pack) were also obtained with these WLEDs: 256.4 lm/W<η_(pack)<273.1lm/W for

4000 K<CCT<4900 K. (2) Using a UCSB Blue LED Chip

The LED die 1902 may be any blue LED, for example a University ofCalifornia, Santa Barbara (UCSB) grown blue LED chip. The different blueLED lamp was formed by attaching an LED die 1902, grown by MOCVD andprocessed at UCSB, onto a silver header 1904 by using a non-conductiveresin (commercially available from GE. The LED die 1902 wastop-emitting, therefore both n and p contacts of the LED 1902 wereconnected by gold wires 1906, 1912 to the posts 1908, 1910 of the silverheader 1904 via wire-bonding. At this stage, the total radiant fluxΦ_(B) of the bare LED die 1902 on the silver header 1904 was measured,using a calibrated integrating sphere, to be 25.3 mW at 20 mA and 4.08 V(DC measurement). The wavelength of light at the emission peak was 444nm.

A transparent silicone from GE was used to form a hemispherical-shapeddome 1914 encapsulating the LED die 1902 on silver header 1904 via amolding technique. The LED die 1902 was placed slightly below (at a ˜2mm distance 1916 from) the center 1918 of the dome 1914. The diameter1920 of the dome was 7.5 mm. The total radiant flux Φ_(B) of theencapsulated blue LED die 1902 on silver header 1904 was then measured,using a calibrated integrating sphere, to be 30.0 mW at 20 mA and 4.08 V(DC measurement). This shows that the transparent silicone dome 1914enabled a 20% increase in total radiant flux as compared to without thedome 1914. The difference between Φ_(B) obtained with the encapsulatedUCSB blue LED chip and Φ_(B) obtained using the commercially availableblue LED chip discussed above in (1), is 25% and is mainly caused by thedifference in LED die 1902 structure.

TABLE 3 Spectral characteristics of WLEDs 1900 comprised of the sameblue LED lamp capped with layers 1922 with three different phosphorconcentrations, wherein the LED die 1902 used in these measurements wasa GaN LED grown and processed at UCSB. Phosphor cap I V CCT conc. (%)(mA) (V) x y (K) CRI 6 5 3.41 .3991 .4653 4102 58.5 6 20 4.08 .3995.4642 4085 58.51 5 5 3.41 .3492 .3785 4946 62.4 5 20 4.08 .3485 .37554960 62.5 3 5 3.41 .2967 .2838 8517 69.5 3 20 4.08 .2960 .3785 8674 69.1

TABLE 4 Main output characteristics of WLEDs 1900 comprised of the sameblue LED lamp capped with layers 1922 with three different phosphorconcentrations, wherein the LED die 1902 used in these measurements wasa GaN LED grown and processed at UCSB. Phosphor cap I V Φ_(B) Φ_(W)F_(W) η_(lum) η_(pack) conc. (%) (mA) (V) (mW) (mW) (lm) (lm/W) (lm/W) 65 3.41 7.3 4.799 1.931 113.3 264.5 6 20 4.08 30.0 19.61 7.974 97.7 265.85 5 3.41 7.3 5.138 1.793 105.5 245.6 5 20 4.08 30.0 21.06 7.402 90.9246.7 3 5 3.41 7.3 5.443 1.550 91.2 212.3 3 20 4.08 30.0 22.14 6.36978.2 212.3

The same phosphor caps 1922 as described in the previous section (1)were used to form WLEDs 1900 using the blue LED lamp comprising the UCSBgrown blue LED die 1902. The main spectral and output characteristics ofthese WLEDs 1900 are given in Table 3 and Table 4.

Again, high overall η_(lum) were obtained with these WLEDs 1900: 105.5lm/W<η_(lum)<113.3 lm/W at 5 mA, and 90.9 lm/W<η_(lum)<97.7 lm/W at 20mA, for 4000 K<CCT<5000 K (a CCT range which is appropriate forwarm-white lighting applications). High η_(pack) were also obtained withthese WLEDs 1900: 245.6 lm/W<η_(pack)<265.8 lm/W for 4000 K<CCT<5000 K.

These high η_(pack) are mainly caused by the fact that the phosphors1922 are in a remote geometry with an air gap 1924 between the phosphorcap 1922 and the transparent dome 1914 encapsulating the LED chip 1902on header 1904. This air gap 1924 is useful to induce total internalreflection inside the phosphor cap 1922 for light emitted by thephosphors 1922. The results could be further improved by using asmoother inner cap surface 1930 and a roughened outer cap surface 1932.The different optical coatings, such as antireflection or dichroiccoatings, were also not integrated with these WLEDs. Furthermore, usinghighly-reflective headers 1904 (which are not necessarily metallic)instead of silver headers 1904 (which tend to oxidize over time), couldalso be beneficial.

The performance of these WLEDs 1900 were compared to the performance ofWLEDs packaged with the phosphors-on-chip configuration 100 of FIG. 1.In the configuration of FIG. 1, using CREE EZR460 chips 104, theobtained η_(pack) were 183.0 lm/W and 206.8 lm/W, for CCTs of 4600 K and4980 K, respectively. In this configuration 100, unlike in the remotephosphor caps configuration 1900, η_(pack) decreases as the phosphorconcentration increases. These η_(pack) values, for thephosphors-on-chip configuration 100, are about 25% lower as compared tothe WLEDs 1900 packaged with remote phosphor caps 1922, for CCTscomparable to the CCT's presented above.

The difference in η_(pack) was not caused by the type of LED die 1902,104 used. A CREE EZR460 chip was used as the chip 1902 and was packagedin the remote phosphor cap configuration 1900 (with the same 6% phosphorconcentration cap 1922 used for the WLEDs 1900 described above). Theresult was % η_(pack)=275.7 lm/W for the device 1900, using a CREEEZR460 chip, for a CCT of 4100 K, which is very similar to the η_(pack)obtained with the CREE EZR450 chip and reported above.

Possible Modifications and Variations

As noted above, FIG. 7 illustrates a WLED, comprising an LED 704 foremitting primary light 724; a secondary emitting species 700,702optically coupled to the LED 704, for emitting secondary light 722, 730,734 comprising one or more wavelengths different than a wavelength ofthe primary light 724; and at least one optically functional interface710, 712, 714, positioned between the secondary emitting species 700 andthe LED 704, wherein the optically functional interface, e.g. 710, 714,is at least partially transparent for the primary light 724 incidentfrom the LED 704 and at least partially reflective for the secondarylight 722, 730 incident from the secondary emitting species 700, 702.The optically functional interfaces 710, 712, 714 may be completelytransparent for the primary 724 light and completely reflective for thesecondary light 722,730. The WLED may comprise more than one opticallyfunctional interface. The optically functional interface, e.g. 714, maybe positioned at a distance 706 b from the LED 704 at least equal to alateral extent 708 of the LED 704.

The secondary emitting species 700, 702 may be a phosphor layer emittingthe secondary light 734, 730, 722 when optically pumped by the primarylight 724. The phosphor may comprise any color emitting phosphor, forexample, a yellow emitting phosphor, and the LED may be any coloremitting LED, for example a blue light emitting LED. The LED may beIII-nitride based, for example.

The optically functional interfaces 710, 712, 714 may have many shapesand comprise many thicknesses or materials, for example, opticalcoatings. For example, the optically functional interface may comprise afirst material 728, or a surface 714 of a first material 728, whereinthe first material 728 may be positioned such that the first material's728 refractive index increases total internal reflection of thesecondary light 730 inside the secondary emitting species 702 at theoptically functional interface 714. The WLED may further comprise asecond material 720 at least partially surrounding the LED 704,positioned such that the second material's 720 refractive index reducesthe primary light's total internal reflection inside the LED 704 at aninterface 742 between the LED 704 and the second material 720; and thefirst material positioned between the secondary emitting species and thesecond material. The first material 728 may be transparent for theprimary light 724 (for example direct LED 704 emission) and reflectivefor the secondary light 730 (secondary emitting species' emission).

The first material's 728 refractive index may be smaller than thephosphor layer's 702 refractive index and the second material's 720refractive index. The first material 728 may have a different refractiveindex from the phosphor layer's 702 refractive index.

FIG. 19 illustrates the second material 1914 may be solid (not hollow)but comprise a dome shaped outer surface 1928, wherein the LED (in theform of an LED die 1902) may be encapsulated by the second material 1914and the header 1904 or LED cup; and the first material 1924 may cap theouter surface 1928 with the phosphor layer 1922 capping the firstmaterial 1924. Throughout the specification, a dome shaped materialrefers to a dome-shape of a surface of the material, and therefore thematerial can be hollow or solid.

A third material 726 may be between at least some of the second material720 and the first material 728, wherein the third material 726 may betransparent for the primary light 724 (or direct emission from the LED704), and reflective for the secondary light 722 emitted by the phosphor702. An optically functional interface 710 may comprise the thirdmaterial 726 or a surface of the third material 726. For example, thethird material 726 may be a layer between the second material 720 andthe first material 728, or a layer or interface inside the secondmaterial 720. For example, the third material 726 may be an optic, thefirst material 728 may be a reflecting medium between the phosphor 702and the optic, and the light extracted from the LED die 704 by thesecond material 720 may be transmitted by the optic towards the phosphor702, and light extracted from the LED die 704 which is reflected awayfrom the phosphor 702 may be back reflected towards the phosphor 702 bythe optic. The optic may be a dichroic mirror, which would provide avery high transmittance for light with wavelengths comprised in thenear-UV to blue range and a high reflectance for light with wavelengthscomprised in the green, yellow, or red portions of the visible spectrum.

FIG. 10 shows the third material may comprise a dome shaped optic 1006,for example capping the second material 1004, wherein an outer surface1006 b of the optic 1006 is dome-shaped or spherical to provide a curvedinterface and the optic 1006 comprises a substantially similarrefractive index to the second material's 1004 refractive index, thefirst material 1012 caps the outer surface 1006 b and the phosphor 1010caps the first material 1012.

FIG. 10 shows how the LED 1000 may be positioned in the LED cup 1002such that the LED's field of view comprises an entirety of the domedshaped outer surface 1006 b. FIG. 17 shows how the LED header 1704 maybe on a mounting fixture 1706, and the phosphor layer 1708 may bemounted to the mounting fixture 1706 via a reflective material 1712capable of reflecting the secondary light.

The present invention further discloses a light source comprising aLight Emitting Diode (LED) packaged/combined with a secondary emittingspecies, wherein an overall luminous efficacy of the light source is 100lm/W or greater and/or a packaging efficiency of the light source is 100lm/W or greater. These figures of merit are not limited to a particularkind of secondary emitting species or LED used. A color rendering indexof the light source may also be 60 or greater, for example.

Thus, FIG. 7 illustrates the present invention discloses a method forfabricating a WLED, comprising providing one or more opticallyfunctional interfaces 710, 712,714, between a secondary emitting species700,702 and an LED 704, to reflect secondary light 722, 730, 734 emittedby the secondary emitting species 700 away from the LED 704, wherein theoptically functional interfaces e.g. 710, 714 are at least partiallytransparent for primary light 724 incident from the LED 704 and at leastpartially reflective for the secondary light incident from the secondaryemitting species 700, and the secondary light 722,730, 734 comprises oneor more wavelengths different than a wavelength of the primary light724.

The present invention is not limited to WLEDs, for example, the presentinvention may also be used to for any light source comprising an LEDcombined with a secondary emitting species, wherein the LED, for examplean LED die, emits one or more primary wavelengths, the secondaryemitting species emits one or more secondary wavelengths, so that thedevice emits one or more different wavelengths.

REFERENCES

The following references are incorporated by reference herein:

-   1. U.S. Pat. No. 5,962,971, issued October 1999, to Sano et al.-   2. U.S. Pat. No. 6,319,425, issued November 2001, to Tasaki et al.-   3. U.S. Pat. No. 6,340,824, issued January 2002, to Komoto et al.-   4. U.S. Pat. No. 6,472,765, issued October 2002, to Chen et al.-   5. U.S. Pat. No. 6,642,652, issued November 2003, to Collins et al.-   6. U.S. Pat. No. 6,917,057, issued July 2005, to Stokes et al.-   7. U.S. Pat. No. 7,005,679, issued February 2006, to Tarsa et al.-   8. U.S. Pat. No. 7,029,935, issued April 2006, to Negley et al.-   9. U.S. Pat. No. 7,157,839, issued January 2007, to Ouderkirk et al.-   10. U.S. Pat. No. 7,180,240, issued February 2007, to Noguchi et al.-   11. U.S Patent Publication No. 2003/0030060, published February    2003, by Okazaki.-   12. U.S Patent Publication No. 2005/0221518, published October 2005,    by Andrews et al.-   13. N. Narendran, Y. Gu, J. P. Freyssinier-Nova, and Y. Zhu,    “Extracting phosphor-scattered photons to improve white LED    efficiency,” Physica Status Solidi A—Applications and Materials    Science 202(6), R60-R62 (2005).-   14. H. Luo, J. K. Kim, E. F. Schubert, J. Cho, C. Sone, and Y. Park,    “Analysis of high-power packages for phosphor-based    white-light-emitting diodes,” Applied Physics Letters 86(24) (2005).-   15. J. K. Kim, H. Luo, E. F. Schubert, J. H. Cho, C. S. Sone,    and Y. J. Park, “Strongly enhanced phosphor efficiency in GaInN    white light-emitting diodes using remote phosphor configuration and    diffuse reflector cup,” Japanese Journal Of Applied Physics Part    2-Letters & Express Letters 44(20-23), L649-L651 (2005).-   16. H. Masui, S. Nakamura, and S. P. DenBaars, “Effects of phosphor    application geometry on white light-emitting diodes,” Japanese    Journal of Applied Physics Part 2-Letters & Express Letters    45(33-36), L910-L912 (2006).

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A method for fabricating a light emitting device, comprising:providing a phosphors layer that is optically coupled to a lightemitting diode (LED), wherein the LED emits a primary light and thephosphors layer emits a secondary light when optically pumped by theLED; wherein a plurality of optically functional or modified interfacesare positioned between the LED and the phosphors layer and between thephosphors layer and an outer or external medium; and wherein at leastone of the interfaces located between the phosphors layer and the LEDreflects the secondary light emitted by the phosphors layer andtransmits the primary light emitted by the LED.
 2. The method of claim1, wherein at least one of the interfaces comprises a surface of a firstmaterial with a refractive index that increases total internalreflection of the secondary light inside the phosphors layer.
 3. Themethod of claim 2, wherein: a second material at least partiallysurrounds the LED and is positioned such that the second material'srefractive index reduces the primary light's total internal reflectioninside the LED at an interface between the LED and the second material;and the first material is positioned between the phosphors layer and thesecond material.
 4. The method of claim 3, wherein the first material'srefractive index is smaller than the phosphors layer's refractive index.5. The method of claim 3, wherein: the LED comprises an LED die on aheader or cup; the second material has a dome shaped outer surface,wherein the LED die is encapsulated by the second material and theheader or cup; and the first material caps the outer surface and thephosphors layer caps the first material.
 6. The method of claim 5,wherein the first material is an air gap having a thickness larger thana wavelength of the primary light.
 7. The method of claim 6, wherein thesecond material is a resin, epoxy, silicon or glass.
 8. The method ofclaim 7, wherein the LED resides on a mounting fixture, and thephosphors layer is attached to the mounting fixture by a reflectivematerial.
 9. The method of claim 3, wherein at least one of theinterfaces comprises a third material that is positioned between atleast some of the second material and the first material.
 10. The methodof claim 9, wherein: the LED comprises an LED die on a header or cup;the LED die is encapsulated by the second material and the header orcup; the third material has a dome shaped outer surface; the firstmaterial caps the outer surface and the phosphors layer caps the firstmaterial; and the LED is positioned in the LED cup such that the LED'sfield of view comprises an entirety of the outer surface of the thirdmaterial and the LED cup reflects the primary light towards thephosphors layer.
 11. A light emitting apparatus, comprising: a phosphorslayer that is optically coupled to a light emitting diode (LED), whereinthe LED emits a primary light, and the phosphors layer emits a secondarylight when optically pumped by the LED; wherein a plurality of opticallyfunctional or modified interfaces are positioned between the LED and thephosphors layer and between the phosphors layer and an outer or externalmedium; and wherein at least one of the interfaces located between thephosphors layer and the LED reflects the secondary light emitted by thephosphors layer and transmits the primary light emitted by the LED. 12.The apparatus of claim 11, wherein at least one of the interfacescomprises a surface of a first material with a refractive index thatincreases total internal reflection of the secondary light inside thephosphors layer.
 13. The apparatus of claim 12, wherein: a secondmaterial at least partially surrounds the LED and is positioned suchthat the second material's refractive index reduces the primary light'stotal internal reflection inside the LED at an interface between the LEDand the second material; and the first material is positioned betweenthe phosphors layer and the second material.
 14. The apparatus of claim13, wherein the first material's refractive index is smaller than thephosphors layer's refractive index.
 15. The apparatus of claim 13,wherein: the LED comprises an LED die on a header or cup; the secondmaterial has a dome shaped outer surface, wherein the LED die isencapsulated by the second material and the header or cup; and the firstmaterial caps the outer surface and the phosphors layer caps the firstmaterial.
 16. The apparatus of claim 15, wherein the first material isan air gap having a thickness larger than a wavelength of the primarylight.
 17. The apparatus of claim 16, wherein the second material is aresin, epoxy, silicon or glass.
 18. The apparatus of claim 17, whereinthe LED resides on a mounting fixture, and the phosphors layer isattached to the mounting fixture by a reflective material.
 19. Theapparatus of claim 13, wherein at least one of the interfaces comprisesa third material that is positioned between at least some of the secondmaterial and the first material.
 20. The apparatus of claim 19, wherein:the LED comprises an LED die on a header or cup; the LED die isencapsulated by the second material and the header or cup; the thirdmaterial has a dome shaped outer surface; the first material caps theouter surface and the phosphors layer caps the first material; and theLED is positioned in the LED cup such that the LED's field of viewcomprises an entirety of the outer surface of the third material and theLED cup reflects the primary light towards the phosphors layer.
 21. Amethod of emitting light, comprising: emitting a primary light from alight emitting diode (LED); and emitting a secondary light from aphosphors layer optically coupled to the LED, when the phosphors layeris optically pumped by the LED; wherein a plurality of opticallyfunctional or modified interfaces are positioned between the LED and thephosphors layer and between the phosphors layer and an outer or externalmedium; and wherein at least one of the interfaces located between thephosphors layer and the LED reflects the secondary light emitted by thephosphors layer and transmits the primary light emitted by the LED.